The present invention relates generally to the field of avionics for collision avoidance systems (CAS). More specifically, the present invention relates generally to displays for use with airborne traffic alert and collision avoidance systems and transponders in formation flight.
Spurred by the collision of two airliners over the Grand Canyon in 1956, the airlines initiated a study of collision avoidance concepts. By the late 1980's, a system for airborne collision avoidance was developed with the cooperation of the airlines, the aviation industry, and the Federal Aviation Administration (FAA). The system, referred to as Traffic Alert and Collision Avoidance System II (TCAS II) was mandated by Congress to be installed on most commercial aircraft by the early 1990's. A chronology of the development of airborne collision avoidance systems can be found in "Introduction to TCAS II," printed by the Federal Aviation Administration of the U.S. Department of Transportation, March 1990.
The development of an effective airborne CAS has been the goal of the aviation community for many years. Airborne collision avoidance systems provide protection from collisions with other aircraft and are independent of ground based air traffic control. As is well appreciated in the aviation industry, avoiding such collisions with other aircraft is a very important endeavor. Furthermore, collision avoidance is a problem for both military and commercial aircraft alike. In addition, a large, simultaneous number of TCAS interrogations from close-in formation aircraft members generate significant radio frequency (RF) interference and could potentially degrade the effectiveness of maintaining precise position/separation criteria with respect to other aircraft and obstacles. Therefore, an additional collision avoidance mode for use in close formation flight with other aircraft is highly desirable.
In addition the problems described above, it is desirable that aircraft, specifically military aircraft, perform precision airdrops, rendezvous, air refueling, and air-land missions at night and in all weather conditions, including Instrument Meteorological Conditions (IMC) with a low probability of detection. Also, it is desirable that these aircraft be allowed to fly in formations including as few as 3 through as many as 250 aircraft to maintain formation position and separation at selectable ranges from 500-ft to 100-nm at all Instrument Flight Rules (IFR) altitudes as described in the Defense Planning Guidelines. Also, the CAS system is to be compatible (primarily because of cost issues) with current station keeping equipment (SKE) systems or they will not be able to fly IMC formation with SKEequipped aircraft.
Referring to FIG. 1, there is shown a block diagram of a conventional TCAS system. Shown in FIG. 1 are TCAS directional antenna 10, TCAS omnidirectional antenna 11, and TCAS computer unit 12, which includes receiver 12A, transmitter 12B, and processor 12C. Also shown are aural annunciator 13, traffic advisory (TA) display 14, and resolution advisory displays 15. Alternatively, the TA and RA displays are combined into one display (not shown). The transponder is comprised of transponder unit 16A, control panel 16B, and transponder antennas 16C and 16D. The TCAS and transponder operate together to function as a collision avoidance system. Those skilled in the art understand that this is merely illustrative of a conventional TCAS. For example, many other configurations are possible such as replacing omni-directional antenna 11 with a directional antenna as is known to those skilled in the art. The operation of TCAS and its various components are well known to those skilled in the art and are not necessary for understanding the present invention.
In a TCAS system, both the interrogator and transponder are airborne and provide a means for communication between aircraft. The transponder responds to the query by transmitting a reply that is received and processed by the interrogator. Generally, the interrogator includes a receiver, an analog to digital converter (A/D), a video quantizer, a leading edge detector, and a decoder. The reply received by the interrogator consists of a series of information pulses which may identify the aircraft, or contain altitude or other information. The reply is a pulse position modulated (PPM) signal that is transmitted in either an Air Traffic Control Radar Beacon System (ATCRBS) format or in a Mode-Select (Mode-S) format.
A TCAS II equipped aircraft can monitor other aircraft within approximately a 20 mile radius of the TCAS II equipped aircraft. (U.S. Pat. No. 5,805,111, Method and Apparatus for Accomplishing Extended Range TCAS, describes an extended range TCAS.) When an intruding aircraft is determined to be a threat, the TCAS II system alerts the pilot to the danger and gives the pilot bearing and distance to the intruding aircraft. If the threat is not resolved and a collision or near miss is probable, then the TCAS II system advises the pilot to take evasive action by, for example, climbing or descending to avoid a collision.
In the past, systems in addition to those described above have been developed to provide collision avoidance for aircraft flying in formation. One type of system is provided by AlliedSignal Aerospace and is known as Enhanced Traffic Alert Collision Avoidance System (ETCAS). The ETCAS provides a normal collision avoidance and surveillance, and a formation/search mode for military specific missions.
The AlliedSignal ETCAS falls short in several ways. First, once an aircraft joins the formation, the ETCAS does not itself or in conjunction with any other onboard system maintain aircraft position and separation within the formation. The ETCAS is simply a situational awareness tool that designates formation members by receiving the Mode 3/A code transmitted from the plane's transponder; the ETCAS does not interface with other aircraft systems to compensate for formation position errors. The ETCAS is actually an aircraft formation member identification and rendezvous system that falls short as a true intra-formation positioning collision avoidance system. Second, the ETCAS Vertical Speed Indicator/Traffic Resolution Alert (VSI/TRA) display does not annunciate relative velocity (range-rate) of the lead formation and member aircraft. The ETCAS is only marginally effective without relative velocity of formation lead aircraft annunciated on the VSI/TRA display. Hence, the pilot has no relative velocity reference to maintain formation position with the lead aircraft, especially during critical turning maneuvers. Third, the ETCAS formation/search mode technique is wholly based upon active TCAS interrogations. Transponder interrogations and the resulting Mode-S transponder replies significantly increase RF reception interference with a large formation of aircraft and could degrade the effectiveness of maintaining precise position/separation criteria. In addition, the increased composite level of RF severely inhibits a large formation from covertly traversing airspace undetected.
Another problem is presented in previous systems wherein station keeping equipment (SKE) on existing military aircraft can support a formation of only 16 aircraft.